1 frozen pound cake (10 3/4 ounces) in aluminum loaf pan, unthawed, or 1 homemade 4 1/2-by-8 1/2-inch pound cake, room temperature
2 cups raspberry sorbet, softened
1 cup vanilla ice cream, softened
1/2 cup coarsely chopped chocolate wafer cookies
2 large egg whites
Cream of tartar
1/2 cup sugar
1/2 teaspoon pure vanilla extract
1. Remove cake from pan and cut horizontally into 4 slices. Line pan with plastic wrap, leaving 6-inch overhang on 2 sides. Build cake layers in pan in this order: bottom cake slice, 1 cup sorbet, cake slice, 1/2 cup ice cream, cookie crumbs, 1/2 cup ice cream, cake slice, 1 cup sorbet, top cake slice. (If necessary, return ice cream to freezer as you work.) Wrap in plastic and freeze at least 1 hour (or up to 3 days).
2. In a heatproof bowl, lightly whisk together egg whites and pinch of cream of tartar. Set bowl over (not in) a pan of simmering water and whisk until whites are foamy. Slowly whisk in sugar and cook, whisking, until sugar is dissolved (to test, rub some between your fingertips).
3. Remove bowl from heat and, with an electric mixer, beat on medium-high until stiff, glossy peaks form, about 6 minutes. Add vanilla and beat 1 minute. Remove cake from pan, trim sides if desired, and dollop meringue on top.